Multicast of traffic data involves sending the same message to more than one destination, e.g., to multiple wireless terminals. Multicast is an important function in wired networks. As wireless links are becoming part of integrated wireless and wired networks, efficient support of multicast in wireless systems is required
In traditional wireless systems, each active wireless terminal, i.e., mobile unit, is given a dedicated control channel and a dedicated traffic channel on which the mobile and the base station exchange point-to-point control and traffic messages, respectively. A brute force technique of realizing multicast is to transmit the traffic individually in the dedicated traffic channels of all intended wireless terminals. However, the same message is repeatedly transmitted a number of times over the air link, which wastes a significant amount of the wireless channel resource.
In some wireless systems, there is a common control channel in the downlink, which all the wireless terminals monitor. Thus, another technique of realizing multicast is to transmit the multicast message over the common control channel. Unfortunately, in general, the common control channel is designed to transport only system-related information and, therefore, it has only a relatively low data rate. Thus, transmitting multicast traffic data over the common control channel generally takes a very long time and doing so also introduces a long delay in transmitting other normal common control messages. Such long transmission intervals and long delays are extremely undesirable.